Escherichia coli synthesize selenium-tRNA when cultured with 0.1 MuM Se03= or Se04=. The presence of selenium is specific, i.e., the selenium level (about 1 mo1 Se per 70 mo1 tRNA) is not affected by the addition of large excesses of sulfur compounds, additional Se03= or Se04=, or by a mutation which blocks 4-thiouracil synthesis. The selenium is not present as esterified selenocysteine or selenomethionine.